The present invention relates generally to the field of positioning systems and, more particularly, to providing time synchronization to Earth satellite navigation system receivers. The invention is described in terms of the Global Positioning System (GPS) but is applicable to similar systems, such as Glonass.
The present invention makes use of a terrestrial, communications network, such as a cellular communications network. In particular, the use of the Global System for Mobile (GSM) network is instanced.
The operation of the present invention sill be better understood with reference to FIGS. 3 and 4, that respectively describe features of the GPS navigation data message and of the GSM signal control channel, and to FIG. 1, that shows the elements of the disclosed invention.
GPS Fundamentals
A GPS locator determines its position by measuring the transit time of signals transmitted simultaneously from a plurality of satellite-borne beacons in the Navstar satellite constellation, inferring therefrom the ranges from the respective beacons, and, from the known locations of the respective beacons at time of transmission, triangulating the ranges to fix a locator position. Normally, the beacons are satellite-borne, but the system may also include fixed, ground-based beacons at known locations, called “pseudolites”, that emulate the signal structure of the satellites to compensate for limited or no satellite visibility. References to satellites or beacons in the following may be taken also to include pseudolites.
The Navstar constellation consists of 24 satellites orbiting the earth every 12 hours. The positions of all satellites are known at all times. This constellation provides any user with 5–8 visible satellites at any time. Each satellite transmits a continuous, unique pseudo-random noise (PN) sequence, or C/A (coarse acquisition) code, at a chip rate of 1023 kHz and a repetition period of 1023 chips (=1 millisecond). The 1023 chips of the C/A code constitute a frame. The several PN sequences from the satellites are mutually orthogonal and have good correlation properties
A 50 Hz navigation or satellite data message (SDM) is superimposed on the PN sequence of each satellite. This data message includes satellite position and velocity data (Almanac—approximate orbital data parameters for all satellites—and Ephemeris—frequently updated parameters describing short sections of the satellite orbits), clock timing data, and transmit time of week (TOW) data, according to GPS system or standard time. The data is sufficient to enable a GPS receiver to compute the respective positions and velocities of the several satellites at any time.
The SDM consists of a sequence of data frames, 300 in FIG. 3, each having five sub-frames 310. Each sub-frame 310 is, in turn, subdivided into ten words 320, each word containing thirty bits 330. The first two words of each sub-frame 310 are the telemetry word (TLM) 340 and the hand-over word (HOW) 350.
A GPS receiver's processor locks onto a particular visible satellite's signal by correlation with a locally stored copy of that satellite's PN sequence. After lock-on has occurred, the processor demodulates the GPS signal to decode the positioning and time data (ephemeris and TOW) from the GPS carrier signal.
The correlation peaks obtained during continued tracking of the satellites provide times of arrival of the PN sequence frames, as measured by a local clock in the GPS receiver. The differences between an arbitrary reference time and measured times of arrival from each satellite, multiplied by the speed of light, are pseudo-ranges ρ from those satellites to the GPS receiver. Typically, the reference time is the (common) time at which the satellites commenced transmission of their respective PN sequences, as measured by the GPS receiver clock. This clock usually differs from the GPS system clock by an unknown time offset, T0. The pseudo-range ρ is related to the true range R of the respective satellite by ρ=R+cb, where the range offset, cb=cT0, c being the speed of light.
The pseudo-ranges are computed from the correlator output from at least four satellites. Using these computed pseudo-ranges and the known satellite positions at transmit time, a position is fixed by triangulation. Pseudo-ranges to at least four satellites are needed to solve at least four simultaneous equations of the form:|s−r|=ρ−cbwhere                s is the position vector of a satellite and        r=(x,y,z) is the position vector of the GPS receiver, for the three unknown Cartesian coordinates x, y, z, and for cb. The satellites are sufficiently far from the GPS receiver that these equations can be linearized in x, y, and z with no loss of accuracy.        
Thus a standard GPS locator calculates position by solving the equation:|sj(t)−r(t)|=Rj(t)=ρj(t)−cTbwhere:                sj(t) and r(t) are the respective position vectors of the jth satellite 150 (see FIG. 1) of the GPS constellation and mobile unit 140 at transmission time t;        Rj(t) is the absolute distance of mobile unit 140 from the jth satellite 150 at transmission time t;        ρj(t) is the pseudo-range measured to the jth satellite 150 at transmission time t;        c is the velocity of light; and        Tb is the clock bias of local time (mobile clock time) with respect to absolute (GPS system) time, hereafter referred to as standard time.        
ρj(t) is measured by correlating the incoming GPS signal with a locally stored copy of the signal transmitted from the jth satellite 150, thereby deducing the signal transit time from the jth satellite 150, and converting that transit time to distance.
sj(t) is calculated from the demodulated position and timing data superimposed on the PN signal.
A comprehensive account of the GPS system may be found in Understanding GPS: Principles and Applications, Elliott D. Kaplan, ed., Artech House Publications, 1996, which is incorporated by reference for all purposes as if filly set forth herein.
GSM Data Structure
GSM operates on a plurality of carrier frequencies within a reserved frequency band. These frequencies, or a sub-set thereof, are allocated to a base station 110 in each cell of the network. Each carrier frequency is partitioned in time, using a TDMA (Time-Division Multiple-Access) scheme which is illustrated in FIG. 4, into slots or burst periods 440 of 15/26 ms (≈0.577 ms) each containing 156.25 bits. Eight slots, or burst periods, are grouped into a TDMA frame 430 ( 120/26 ms, ≈4.615 ms). Frames 430 are grouped to form multi-frames 420. The number of TDMA frames per multi-frame depends upon Whether the channels are used as traffic channels (26 frames), which are transiently assigned to individual mobile units to carry speech and data, or as network control channels (51 frames). The latter is illustrated in FIG. 4. Super-frames 410 consist of 51 multi-frames 420 (for traffic), or 26 (for control). Hyper-frames 400 contain 2048 super-frames 410 and span 3.4816 hr. A logical channel corresponds to a specific time slot 440 within successive TDMA frames 430.
In the case of signal control channels, lime-slot zero of each TDMA frame forms a logical channel, as described above, and is utilized, inter alia, as a frequency-correction and a synchronization channel (SCH) 450. This serves to synchronize a GSM mobile unit to the time-slot structure of a network cell by defining the boundaries of burst periods and time-slot numbering. Synchronization channel 450, in distinction to traffic channels, is one of a group of common channels accessible to all mobile units. Common channels are defined within a 51-frame multi-frame 420. The present invention, as disclosed, exploits the SCH but other embodiments might utilize other features of the GSM signal structure to the same end.
GSM employs four types of burst, distinguished by internal structure and, in one case, length. The S burst, used on the synchronization channel, consists of 156.25 bits transmitted in a slot length of 0.577 ms.
Because the GSM signal has a known and precisely regulated structure, it is possible to know the time interval between two identified events in that signal.
GPS Operation
The standard GPS process outlined above has severe drawbacks in urban areas, where signal blockage may result in a signal-to-noise ratio too low to allow demodulation of the data message bits (20 ms bit duration) in order to decode the satellite navigation data message and therefrom compute a location, even though the receiver can acquire and track the signals. In this event, the PR may still be measured with sufficient accuracy by operating coherently on longer signal strings (˜100–1000 ms), thus achieving an enhanced signal-to-noise ratio, as taught in co-pending U.S. patent application Ser. No. 09/585,619.
Krasner, in U.S. Pat. No. 5,945,944, Method and apparatus for determining time for GPS receivers, discloses a method for providing GPS standard time to a wireless GPS locator operating within the coverage area of a cell-based communication network The GPS receiver includes an auxiliary receiver suitable for the cell-based network and acquires GPS standard time by communicating measurements of pseudo-ranges and received GPS time indicators to a GPS base station where a position is calculated and transmitted back to the mobile unit, all via the cell-based network. But this method also suffers a drawback in that it requires the addition of a GPS base station connected to the cell-based network.
There is thus a need for, and it would be highly advantageous to have, an alternative method and system of providing GPS standard time to a wireless GPS receiver.